Vertebrate Fauna of North Carolina Coastal Islands: A Study in the Dynamics of Animal Distribution I. Ocracoke Island

Engels, William

The North Carolina "outer banks" are offshore bars, submarine in origin; the present flora and fauna therefore was acquired sometime subsequent to the emergence of the bars. A study of the vertebrate fauna has been undertaken to determine which species have become established on the islands and to analyze the several factors concerned, namely, source of invading species, possible routes of invasion, kinds of invading species, limiting factors in establishment and maintenance of invading species, and degree of differentiation attained, if any. Of these bars, Ocracoke Island is probably now the most nearly inaccesible to invading species. Its exposed position subjects it to periodical violent storms, during which most of the island is inundated, and as a result of which the populations are periodically decimated, and the topography and habitats made unstable. From about 1750, when old Hatteras Inlet closed up, until 1846, when the present Hatteras Inlet was formed, Ocracoke was continuous with Hatteras Island. The new inlet was cut through the bar about five miles northeastward of the former inlet; thus the northeast portion of Ocracoke Island was originally part of Hatteras Island. Except for an irregular line of dunes fronting the sea, the island is almost entirely less than three feet above sea level. The strand is barren, the sand dunes covered with a characteristic grass, "sea oats." Dry thickets are scattered on the inner side of the dunes; back of this area a line of thickets borders the salt marsh, which extends to the shore of the sound; there are only a few small patches of woodland. On this island are found a single amphibian, two species of lizards, at least four snakes, four turtles, seventeen breeding species of land birds, and five or possibly six mammals, two of which are non-native. These make up only about 17% of the total number of species in the fauna of the adjacent mainland. Of species or groups of species found on adjacent islands or mainland, the following probably are missing from Ocracoke: urodeles, Rana; Alligator, Anolis and Eumeces; crotaline snakes; nighthawks, swifts, hummingbirds, woodpeckers, titmice and nuthatches; moles, shrews, squirrels, and native rats and mice. The meagerness of the fauna is ascribed to several factors: relative inaccessibility; geographical relationship to "faunal areas"; absence of certain habitats and small size (total area) of others; and relative instability of favorable habitat conditions. The factor first mentioned limits the number of animals capable of invading the island, while the second determines the precise kinds available for invasion. The third factor limits the establishment of invading species; the last-mentioned factor reduces the certainty of maintenance of species following establishment. The one-time merger of Ocracoke with Hatteras Island and subsequent transfer to the former of part of the latter had little effect if any on the facies of the Ocracoke fauna, as evidenced especially by the absence of several species of mammals now occurring on Hatteras. At least four members of the fauna (roughly twelve per cent) owe their presence on the island to man; of these, one species certainly, and probably a second, are now independent of him, but at least two established species could not maintain themselves there, if he should abandon the island, because of the resultant loss of the edificarian habitat on which they depend.

Vertebrate Fauna of North Carolina Coastal Islands: A Study in the Dynamics of Animal Distribution I. Ocracoke Island

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